Roman Empire and his calendar

The Roman calendar (being a lunar calendar) that was used before 46 BC consisted only 10 months (with the vernal equinox in the first month).

September means "the 7th month"

October means "the 8th month"

November means "the 9th month"

December means "the 10th month"

Until January and February were added before March, people argue that September is not "the 7th month" anymore. Again, keep in mind that the first month is actually the month that consists the vernal equinox, which is March. Therefore, it would be unnecessary to switch back March as the first month and January/February as the 11th/12th because January/February are considered as "winter months" whereas March is considered as a "spring month" (well, at least in the Northern Hemisphere. It would be the opposite for Australia =D).